1. Technical Field
The present invention relates to a method for producing a soft magnetic powdered core, which is preferably used for electric transformers, reactors, thyristor valves, noise filters, choke coils, and the like, and is more preferably used for soft magnetic motor cores, rotors and yokes of motors in home appliances and industrial instruments, solenoid cores (stator cores) for solenoid valves installed in an electronically controlled fuel injector for a diesel engine or a gasoline engine, and the like, which require high magnetic flux density.
2. Background Art
Iron loss is a very important characteristic of soft magnetic cores and is defined by eddy current loss relating to a specific electric resistivity value of a magnetic core and hysteresis loss affected by strain in a soft magnetic powder, which is generated in a production process of the soft magnetic powder and subsequent processing steps. The iron loss W can be specifically defined by the sum of eddy current loss We and hysteresis loss Wh as shown in the following formula (1). The eddy current loss We and the hysteresis loss Wh can be defined by the following formulas (2) and (3), respectively. In this case, “f” represents the frequency, “Bm” represents the exciting magnetic flux density, “ρ” represents the specific electric resistivity value, “t” represents the thickness of a material, and “k1” and “k2” represent coefficients.W=We+Wh  (1)We=(k1Bm2t2/ρ)f2  (2)Wh=k2Bm1.6f  (3)
As is clear from the formulas (1) to (3), while the hysteresis loss Wh is proportional to the frequency f, the eddy current loss We is proportional to the square of the frequency f. Therefore, decrease of the eddy current loss We is effective in decreasing the iron loss W, specifically in a high frequency area. In order to decrease the eddy current loss We, the specific electric resistivity value ρ should be increased by limiting the eddy current loss in a small area.
A soft magnetic powdered core is formed by interposing nonmagnetic resin between soft magnetic powder particles such as iron powders so as to limit eddy current loss to each soft magnetic powder particle. The soft magnetic powdered core has high specific electric resistivity value ρ and small eddy current loss We, and it can be produced by simple methods, whereby it is conventionally widely used (for example, see Japanese Patent Application of Laid-Open No. 60-235412). In the soft magnetic powdered core disclosed in the above Japanese Patent Application of Laid-Open No. 60-235412, resin exists between soft magnetic powder particles, whereby electrical insulation between the soft magnetic powder particles is specifically ensured. As a result, the eddy current loss We is decreased, and the soft magnetic powders are tightly bound, whereby strength of the soft magnetic powdered core is improved.
On the other hand, in a soft magnetic powdered core, nonmagnetic resin exists between soft magnetic powder particles, whereby amount of the soft magnetic powder (space factor) decreases according to the amount of resin contained in the magnetic core. Therefore, the soft magnetic powdered core has a disadvantage in that the magnetic flux density may be decreased. In order to overcome this disadvantage, a technique is disclosed in Japanese Patent Application of Laid-Open No. 9-320830 in which electrical insulation of a soft magnetic powder is improved by forming an insulating film on surfaces of the soft magnetic powder particles so as to decrease additive amount of resin, and this technique is used in practice. Moreover, further improvement in the magnetic properties is required recently, and in response to this requirement, a soft magnetic powdered core is disclosed in Japanese Patent Application of Laid-Open No. 2004-146804 in which additive amount of resin is further decreased.
As described above, the additive amount of resin in a soft magnetic powdered core is required to be small from the viewpoint of the magnetic properties. However, the soft magnetic powdered core has a structure in which the resin binds the soft magnetic powder particles, and the reduction of the additive amount of resin thereby causes a decrease in strength of the soft magnetic powdered core. The soft magnetic powdered core was not used for a member that requires strength, and the decrease in the strength was not a serious problem. On the other hand, recently, a portion is required to have a highly precise and complex shape, and a soft magnetic powdered core should be machined. Under such circumstances, it is difficult to machine a soft magnetic powdered core in which the additive amount of resin is further decreased, because strength thereof is not sufficient. A soft magnetic powdered core may be used in combination as various actuators, or it may be molded in resin, and therefore, external force is often applied thereto. Moreover, chipping easily occurs when soft magnetic powdered cores strike each other during a process such as when they are being conveyed, and the soft magnetic powdered core requires extra attention during assembling and when being transported. In order to prevent chipping of the soft magnetic powdered core, increase in the binding power of the soft magnetic powdered core is required.